Fluid pump systems, and specifically oil pump systems, are well known in the art. In a typical oil pump system, the oil pump is driven by an engine s crankshaft and is either located on the front of the engine or in the oil pan. Because the oil pump is driven by the crankshaft, it runs at a fixed speed ratio to the engine crankshaft, which ratio is determined by the flow volume required to maintain oil pressure at low speeds. This ratio produces excessive flow volume, however, which may result in significant energy loss, at higher engine speeds. Moreover, if the oil pump is located on the front of the engine, enough space must be provided to accommodate it.
The use of dual engine balance shafts for certain engines are known in the art to aid in balancing engine vibration and in reducing engine noise. Examples of the use of dual engine balance shafts are disclosed in U.S. Pat. No. 4,703,724 assigned to Chrysler Motors Corporation and U.S. Pat. No. 5,535,643 assigned to General Motors Corporation. In operation, the balance shafts are connected to the engine crankshaft in such a way as to rotate at twice the crankshaft speed. The two balance shafts also rotate in opposite directions to cancel each other's lateral unbalance. The balance shafts counterbalance the vertical shaking forces caused by the acceleration and deceleration of the reciprocating piston assemblies and connection rods.
One problem with the use of balance shafts is that the firing and compression strokes alternately accelerate and decelerate the crankshaft s rotation. These angular accelerations of the crankshaft occur at all engine speeds. However, the “Rigid Body Motion” angular displacements which result are greatest at low speeds, where the capacity for kinetic energy storage (a function of the square of velocity) by the engine s rotating inertia is low, and the time duration of the acceleration phases are high.
This Rigid Body Motion which is greatest at low speed engine operation can create gear rattle by alternately speeding up and slowing down the input shaft of the two counter-rotating balance shafts. The meshing clearance or backlash between the teeth of the two gears opens and then closes noisily, while the balance shafts attempt to maintain constant rotational speed by virtue of their inertia.
In an effort to reduce these vibrational and noise problems, coupling a single oil pump to an engine balance shaft is known. However, these efforts have resulted in inefficient systems that utilize more engine power than is necessary causing decreased fuel efficiency. Moreover, because of the increased engine power usage from excess pump flow volume, the engine can generate more noise and oil temperature than is desired as it drives the oil pump.
While it is known from general pumping technology to interconnect two or more pumps by a fluid control valve, the cost-effective utilization of a low speed supplemental pump to control the low speed problem of gear rattle in a twin balance shaft system is not. Examples of such general pumping technology are shown in U.S. Pat. Nos. 4,306,840, 4,245,964, and 4,832,579. These general pumping technologies also fail to achieve maximum energy efficiency because they discharge the output of the switched pump past a one-way valve to a common inlet manifold, which is operating at below atmospheric pressure to lift oil from the oil pan or oil sump.